Wedge brake system for propeller rotor

ABSTRACT

Aspects of the technology relate to a braking assembly for a lateral propulsion system of a high altitude platform (HAP) configured to operate in the stratosphere. Power is supplied to a propeller assembly as needed during lateral propulsion so that the HAP can move to a desired location or remain on station. When lateral propulsion is not needed, power is no longer supplied to the propeller assembly and it may slowly cease rotating. However, in certain situations, it may be necessary to cause the propeller assembly to stop rotating as soon as possible. This can include an unplanned descent. Rapid braking can avoid the propeller blades from entangling in the envelope, parachute or other parts of the HAP. A reusable brake is employed to prevent uncontrolled rotation of the propeller on descent, or otherwise to prevent the propeller from spinning freely when not being used to propel the HAP laterally.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. application Ser. No. ______,entitled Wedge Brake Control System and Method, attorney docket No. LOON3.0F-2227 II [9143], filed concurrently herewith, the entire disclosureof which is incorporated herein by reference.

BACKGROUND

Telecommunications connectivity via the Internet, cellular data networksand other systems is available in many parts of the world. However,there are locations where such connectivity is unavailable, unreliableor subject to outages from natural disasters. Some systems may providenetwork access to remote locations or to locations with limitednetworking infrastructure via satellites or high altitude platforms. Inthe latter case, due to environmental conditions and other limitations,it is challenging to keep the platforms aloft and operational over adesired service area for long durations, such as weeks, months or more.

SUMMARY

Aspects of the technology relate to a high altitude platform (HAP) thatis able to remain on station or move in a particular direction toward adesired location, for instance to provide telecommunication services.The high altitude platform may be a lighter-than-air platform such as aballoon configured to operate in the stratosphere. For instance, theballoon may include an envelope filled with lift gas and a payload forproviding telecommunication services, with a connection member couplingthe payload with the envelope. A lateral propulsion system may providedirectional thrust for moving the balloon toward a destination orremaining on station. This can include a pointing mechanism that alignsa propeller assembly of the lateral propulsion system along a certainheading. By way of example, the propeller assembly may be able to rotateup to 360° or more around the connection member in order to adjust theballoon's heading.

During operation, the propeller is pointed along a specified heading androtates at a particular velocity (e.g., hundreds or thousands ofrevolutions per minute). Power is supplied to the propeller as neededduring lateral propulsion. When lateral propulsion is not needed, poweris no longer supplied to the propeller and it may slowly cease rotating.However, in certain circumstances such as a catastrophic envelopefailure or loss of overall system power, it may be necessary to causethe propeller to cease rotating immediately. This can be done, forinstance, in an unplanned descent scenario to avoid a rotating propellerblade from entangling in the envelope, parachute or other parts of thehigh altitude platform. Aspects of the technology implement a reusablewedge brake to prevent uncontrolled rotation of the propeller ondescent, e.g., due to dynamic pressure, or otherwise to prevent thepropeller from spinning freely when not being used to propel the HAPlaterally.

According to one aspect, a brake mechanism is configured to stoprotation of a propeller assembly. The brake mechanism comprises a brakepad configured to engage with a hub portion of the propeller assembly tostop the rotation of the propeller assembly, a housing receiving atleast part of the brake pad therein during disengagement from thepropeller assembly, a brake sensor configured to detect whether thebrake pad is engaged with or disengaged from the hub portion of thepropeller assembly, and an actuator assembly including a magnet and anactuator. The actuator assembly is configured to maintain the brake padin a disengaged state during a first operational mode and to cause thebrake pad to engage the hub portion of the propeller assembly during asecond operational mode.

In one example, the actuator assembly further includes a spring couplingthe brake pad to the actuator. The spring is configured to provide aspring force to cause the brake pad to engage the hub portion of thepropeller assembly during the second operational mode. Here, theactuator assembly may further include an arm member coupled between theactuator and the spring. In this case, the brake mechanism may have acable affixed to the brake pad and the arm member, wherein the spring isdisposed around at least part of the cable. The arm member may include afirst extension affixed to the magnet, a second extension opposite thefirst extension, and a central region disposed between the firstextension and the second extension, the central region affixed to theactuator. Here, the brake mechanism may further comprise a second magnetaffixed to the second extension, and a Hall Effect sensor in operativecommunication with the second magnet affixed to the second extension, inwhich the Hall Effect sensor is configured to detect the strength of amagnetic field associated with the second magnet to detect a relativedisplacement of the brake pad.

The actuator may be a solenoid. The housing may include a housingbracket and a cover section affixed to the housing bracket, where thehousing bracket and the cover section define a receptacle to at leastpartly receive the brake pad. In this case, the housing bracket may havean arcuate side configured to abut a motor assembly that drives thepropeller assembly.

According to another aspect, a propulsion system is provided for usewith a high altitude platform configured to operate in the stratosphere.The propulsion system comprises a propeller assembly including aplurality of propeller blades and central propeller hub affixed to theplurality of propeller blades, a motor assembly, and a brake mechanism.The motor assembly is operatively coupled to the central propeller hubof the propeller assembly. The motor assembly is configured to actuatethe propeller assembly to drive the high altitude platform in a lateraldirection in the stratosphere. The brake mechanism is configured to stoprotation of the propeller assembly. The brake mechanism comprises abrake pad configured to engage with the propeller hub of the propellerassembly to stop the rotation of the propeller assembly, a housingreceiving at least part of the brake pad therein during disengagementfrom the propeller assembly, a brake sensor configured to detect whetherthe brake pad is engaged with or disengaged from the propeller hub ofthe propeller assembly, and an actuator assembly including a magnet andan actuator. The actuator assembly is configured to maintain the brakepad in a disengaged state during a first operational mode and to causethe brake pad to engage the propeller hub of the propeller assemblyduring a second operational mode.

The actuator assembly may further include a spring coupling the brakepad to the actuator. The spring is configured to provide a spring forceto cause the brake pad to engage the propeller hub of the propellerassembly during the second operational mode. Here, the actuator assemblymay further include an arm member coupled between the actuator and thespring. The brake mechanism may include a cable affixed to the brake padand the arm member, wherein the spring is disposed around at least partof the cable. The arm member may include a first extension affixed tothe magnet, a second extension opposite the first extension, and acentral region disposed between the first extension and the secondextension, the central region affixed to the actuator. In one scenario,the propulsion system further comprises a second magnet affixed to thesecond extension, and a Hall Effect sensor in operative communicationwith the second magnet affixed to the second extension, in which theHall Effect sensor is configured to detect a strength of a magneticfield associated with the second magnet to detect a displacement of thebrake pad relative to the propeller hub.

The actuator may be a solenoid. In another example the housing includesa housing bracket and a cover section affixed to the housing bracket.Here, the housing bracket and the cover section define a receptacle toat least partly receive the brake pad. The housing bracket may includean arcuate side configured to abut a housing of the motor assembly.

According to a further aspect, a high altitude platform is provided,which is configured to operate in the stratosphere. The high altitudeplatform comprises a balloon envelope, a payload, a connecting membercoupling the payload to the balloon envelope, and the propulsion systemdescribed above, wherein the propulsion system is operatively engagedwith the connecting member. In one example, the high altitude platformfurther comprising a control system configured to actuate the brakemechanism for engagement with and disengagement from the propeller hubin response to a predetermined condition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional diagram of an example system in accordance withaspects of the disclosure.

FIG. 2 illustrates a balloon configuration in accordance with aspects ofthe disclosure.

FIG. 3 is an example payload arrangement in accordance with aspects ofthe disclosure.

FIG. 4 is an example of a balloon platform with lateral propulsion inaccordance with aspects of the disclosure.

FIGS. 5A-B illustrate an example lateral propulsion system according toaspects of the technology.

FIGS. 6A-C illustrate a control assembly in accordance with aspects ofthe technology.

FIG. 7 is a block diagram of an example electronics module in accordancewith aspects of the disclosure.

FIG. 8 illustrates an exemplary lateral propulsion controllerarrangement in accordance with aspects of the technology.

FIG. 9 illustrates a braking example in accordance with aspects of thedisclosure.

FIGS. 10A-C illustrate a brake mechanism in accordance with aspects ofthe disclosure.

FIGS. 11A-G illustrate portions of the helical brake mechanism inaccordance with aspects of the technology.

FIG. 12 illustrates a brake pad arrangement in accordance with aspectsof the technology.

FIG. 13 illustrates an arm member in accordance with aspects of thetechnology.

FIG. 14 illustrates a portion of a brake mechanism in accordance withaspects of the technology.

FIG. 15A-B illustrate state and flow diagrams of an engagement anddisengagement approach in accordance with aspects of the technology.

FIG. 16 illustrates a plot of torque and velocity versus time inaccordance with aspects of the technology.

FIG. 17 illustrates a method according to aspects of the technology.

FIG. 18 illustrates another method according to aspects of thetechnology.

DETAILED DESCRIPTION Overview

The technology relates to a brake system for a propeller assembly, suchas may be used in lateral propulsion systems for HAPs designed tooperate, e.g., in the stratosphere. Stratospheric HAPs, such as highaltitude balloons and other lighter-than-air craft, may have a floataltitude of between about 50,000-120,000 feet above sea level. Theambient temperature may be on the order of −10° C. to −90° C. or colder,depending on the altitude and weather conditions. These and otherenvironmental factors in the stratosphere can be challenging for lateralpropulsion systems.

Under typical operating conditions, the propeller assembly may only bepowered for a certain period of time. When the power is turned off, thepropeller assembly will slow down and eventually stop. However, incertain emergent situations it may be necessary to immediately cause thepropeller assembly to stop rotating, such as during descent of the HAP.The brake system and methods discussed herein are designed to enablerapid braking of the propeller assembly in emergent situations, withoutaffecting propeller operation otherwise.

As explained below, an example lateral propulsion system may employ apropeller assembly to provide directional adjustments to the HAP, forinstance to counteract movement due to the wind, or to otherwise causethe HAP to move along a selected heading. Such adjustments can enhanceoperation across a fleet of HAPs. For instance, by employing a smallamount of lateral propulsion at particular times, a given platform mayremain on station over a desired service area for a longer period thatwithout such propulsion, or change direction to move towards aparticular destination. The platform may also be able to return to thedesired service area more quickly using lateral propulsion to compensateagainst undesired wind effects. Applying this approach for some or allof the platforms in the fleet may mean that the total number ofplatforms necessary to provide a given level of service (e.g.,telecommunications coverage for a service area) may be significantlyreduced as compared to a fleet that does not employ lateral propulsion.

One or more motors can be used to actuate a lateral propulsion system ofthe HAP to effect the directional changes. This can include a pointingaxis motor for rotating the lateral propulsion system to a particularheading, and a drive motor for causing a propeller assembly or otherpropulsion mechanism to turn on and off. In one example, a controller ofthe lateral propulsion system is configured to cause the pointing axismotor to rotate the propeller assembly about a connection member of theHAP by up to 360° or more. The drive motor may be actuated to provide aselected amount of propulsion, which may be based on the size of thepropeller assembly and the speed of rotation. Depending on the mode ofoperation, the propeller assembly may rotate at speeds in excess of 2500rpm. Once the drive motor is disengaged, it may take tens of seconds,minutes or longer for the propeller assembly to slow down and eventuallystop rotating. Using a brake assembly enables the lateral propulsionsystem to cause the propeller assembly to quickly stop rotating, forinstance within 0.3-2.0 seconds, without damaging the propellerassembly. This can be particularly beneficial in situations such as arapid or uncontrolled descent, for instance to avoid entanglement with aparachute. By way of example, the brake can be used to prevent thepropeller from spinning uncontrollably during descent due to dynamicpressure, e.g., when the propeller assembly is being back-driven becausethe propeller assembly is falling through increasingly dense air as theballoon descends. Back-driving an electric propeller motor can causeovervoltage in electronics and damage to the motor bearings orcomponents. Stopping the propeller may not eliminate entanglement risk,but would prevent a rotating propeller from “grabbing” or “wrapping” ofcomponents such as parachutes. The braking arrangement can also behelpful to prevent the propeller from free spinning when not in use,which could generate heat and/or power at the propeller motor (and apotential overvoltage situation).

Example Balloon Systems

FIG. 1 depicts an example system 100 in which a fleet of high altitudeplatforms, such as balloon HAPs, may be used. This example should not beconsidered as limiting the scope of the disclosure or usefulness of thefeatures described herein. System 100 may be considered a balloonnetwork. In this example, balloon network 100 includes a plurality ofdevices, such as balloons 102A-F as well as ground-base stations 106 and112. Balloon network 100 may also include a plurality of additionaldevices, such as various computing devices (not shown) as discussed inmore detail below or other systems that may participate in the network.

The devices in system 100 are configured to communicate with oneanother. As an example, the balloons may include communication links 104and/or 114 in order to facilitate intra-balloon communications. By wayof example, links 114 may employ radio frequency (RF) signals (e.g.,millimeter wave transmissions) while links 104 employ free-space opticaltransmission. Alternatively, all links may be RF, optical, or a hybridthat employs both RF and optical transmission. In this way balloons102A-F may collectively function as a mesh network for datacommunications. At least some of the balloons may be configured forcommunications with ground-based stations 106 and 112 via respectivelinks 108 and 110, which may be RF and/or optical links.

In one scenario, a given balloon 102 may be configured to transmit anoptical signal via an optical link 104. Here, the given balloon 102 mayuse one or more high-power light-emitting diodes (LEDs) to transmit anoptical signal. Alternatively, some or all of the balloons 102 mayinclude laser systems for free-space optical communications over theoptical links 104. Other types of free-space communication are possible.Further, in order to receive an optical signal from another balloon viaan optical link 104, the balloon may include one or more opticalreceivers.

The balloons may also utilize one or more of various RF air-interfaceprotocols for communication with ground-based stations via respectivecommunication links. For instance, some or all of balloons 102A-F may beconfigured to communicate with ground-based stations 106 and 112 via RFlinks 108 using various protocols described in IEEE 802.11 (includingany of the IEEE 802.11 revisions), cellular protocols such as GSM, CDMA,UMTS, EV-DO, WiMAX, and/or LTE, 5G and/or one or more proprietaryprotocols developed for long distance communication, among otherpossibilities.

In some examples, the links may not provide a desired link capacity forballoon-to-ground communications. For instance, increased capacity maybe desirable to provide backhaul links from a ground-based gateway.Accordingly, an example network may also include downlink balloons,which could provide a high-capacity air-ground link between the variousballoons of the network and the ground-base stations. For example, inballoon network 100, balloon 102F may be configured as a downlinkballoon that directly communicates with station 112.

Like other balloons in network 100, downlink balloon 102F may beoperable for communication (e.g., RF or optical) with one or more otherballoons via link(s) 104. Downlink balloon 102F may also be configuredfor free-space optical communication with ground-based station 112 viaan optical link 110. Optical link 110 may therefore serve as ahigh-capacity link (as compared to an RF link 108) between the balloonnetwork 100 and the ground-based station 112. Downlink balloon 102F mayadditionally be operable for RF communication with ground-based stations106. In other cases, downlink balloon 102F may only use an optical linkfor balloon-to-ground communications. Further, while the arrangementshown in FIG. 1 includes just one downlink balloon 102F, an exampleballoon network can also include multiple downlink balloons. On theother hand, a balloon network can also be implemented without anydownlink balloons.

A downlink balloon may be equipped with a specialized, high bandwidth RFcommunication system for balloon-to-ground communications, instead of,or in addition to, a free-space optical communication system. The highbandwidth RF communication system may take the form of an ultra-widebandsystem, which may provide an RF link with substantially the samecapacity as one of the optical links 104.

In a further example, some or all of balloons 102A-F could be configuredto establish a communication link with space-based satellites and/orother types of high altitude platforms (e.g., drones, airplanes,airships, etc.) in addition to, or as an alternative to, a ground basedcommunication link. In some embodiments, a balloon may communicate witha satellite or a high altitude platform via an optical or RF link.However, other types of communication arrangements are possible.

As noted above, the balloons 102A-F may collectively function as a meshnetwork. More specifically, since balloons 102A-F may communicate withone another using free-space optical links, the balloons maycollectively function as a free-space optical mesh network. In amesh-network configuration, each balloon may function as a node of themesh network, which is operable to receive data directed to it and toroute data to other balloons. As such, data may be routed from a sourceballoon to a destination balloon by determining an appropriate sequenceof links between the source balloon and the destination balloon.

The network topology may change as the balloons move relative to oneanother and/or relative to the ground. Accordingly, the balloon network100 may apply a mesh protocol to update the state of the network as thetopology of the network changes. For example, to address the mobility ofthe balloons 102A to 102F, the balloon network 100 may employ and/oradapt various techniques that are employed in mobile ad hoc networks(MANETs). Other examples are possible as well.

Balloon network 100 may also implement station-keeping functions usingwinds and altitude control and/or lateral propulsion to help provide adesired network topology. For example, station-keeping may involve someor all of balloons 102A-F maintaining and/or moving into a certainposition relative to one or more other balloons in the network (andpossibly in a certain position relative to a ground-based station orservice area). As part of this process, each balloon may implementstation-keeping functions to determine its desired positioning withinthe desired topology, and if necessary, to determine how to move toand/or maintain the desired position. Alternatively, the platforms maybe moved without regard to the position of their neighbors, for instanceto enhance or otherwise adjust communication coverage at a particulargeographic location.

The desired topology may thus vary depending upon the particularimplementation and whether or not the balloons are continuously moving.In some cases, balloons may implement station-keeping to provide asubstantially uniform topology where the balloons function to positionthemselves at substantially the same distance (or within a certain rangeof distances) from adjacent balloons in the balloon network 100.Alternatively, the balloon network 100 may have a non-uniform topologywhere balloons are distributed more or less densely in certain areas,for various reasons. As an example, to help meet the higher bandwidthdemands, balloons may be clustered more densely over areas with greaterdemand (such as urban areas) and less densely over areas with lesserdemand (such as over large bodies of water). In addition, the topologyof an example balloon network may be adaptable allowing balloons toadjust their respective positioning in accordance with a change in thedesired topology of the network.

The balloons of FIG. 1 may be high-altitude balloons that are deployedin the stratosphere. As an example, in a high altitude balloon network,the balloons may generally be configured to operate at stratosphericaltitudes, e.g., between 50,000 ft and 70,000 ft or more or less, inorder to limit the balloons' exposure to high winds and interferencewith commercial airplane flights. In order for the balloons to provide areliable mesh network in the stratosphere, where winds may affect thelocations of the various balloons in an asymmetrical manner, theballoons may be configured to move latitudinally and/or longitudinallyrelative to one another by adjusting their respective altitudes, suchthat the wind carries the respective balloons to the respectivelydesired locations. Lateral propulsion may also be employed to affect theballoon's path of travel.

In an example configuration, the high altitude balloon platforms includean envelope and a payload, along with various other components. FIG. 2is one example of a high-altitude balloon 200, which may represent anyof the balloons of FIG. 1. As shown, the example balloon 200 includes anenvelope 202, a payload 204 and a termination (e.g., cut-down &parachute) device 206.

The envelope 202 may take various shapes and forms. For instance, theenvelope 202 may be made of materials such as polyethylene, mylar, FEP,rubber, latex or other thin film materials or composite laminates ofthose materials with fiber reinforcements imbedded inside or outside.Other materials or combinations thereof or laminations may also beemployed to deliver required strength, gas barrier, RF and thermalproperties. Furthermore, the shape and size of the envelope 202 may varydepending upon the particular implementation. Additionally, the envelope202 may be filled with different types of gases, such as air, heliumand/or hydrogen. Other types of gases, and combinations thereof, arepossible as well. Shapes may include typical balloon shapes like spheresand “pumpkins”, or aerodynamic shapes that are symmetric, provide shapedlift, or are changeable in shape. Lift may come from lift gasses (e.g.,helium, hydrogen), electrostatic charging of conductive surfaces,aerodynamic lift (wing shapes), air moving devices (propellers, flappingwings, electrostatic propulsion, etc.) or any hybrid combination oflifting techniques.

According to one example shown in FIG. 3, a payload 300 of a HAPplatform includes a control system 302 having one or more processors 304and on-board data storage in the form of memory 306. Memory 306 storesinformation accessible by the processor(s) 304, including instructionsthat can be executed by the processors. The memory 306 also includesdata that can be retrieved, manipulated or stored by the processor. Thememory can be of any non-transitory type capable of storing informationaccessible by the processor, such as a hard-drive, memory card, ROM,RAM, and other types of write-capable, and read-only memories. Theinstructions can be any set of instructions to be executed directly,such as machine code, or indirectly, such as scripts, by the processor.In that regard, the terms “instructions,” “application,” “steps” and“programs” can be used interchangeably herein. The instructions can bestored in object code format for direct processing by the processor, orin any other computing device language including scripts or collectionsof independent source code modules that are interpreted on demand orcompiled in advance. The data can be retrieved, stored or modified bythe one or more processors 304 in accordance with the instructions.

The one or more processors 304 can include any conventional processors,such as a commercially available CPU. Alternatively, each processor canbe a dedicated component such as an ASIC, controller, or otherhardware-based processor. Although FIG. 3 functionally illustrates theprocessor(s) 304, memory 306, and other elements of control system 302as being within the same block, the system can actually comprisemultiple processors, computers, computing devices, and/or memories thatmay or may not be stored within the same physical housing. For example,the memory can be a hard drive, memory card or other storage medialocated in a housing different from that of control system 302.Accordingly, references to a processor, computer, computing device, ormemory will be understood to include references to a collection ofprocessors, computers, computing devices, or memories that may or maynot operate in parallel.

The payload 300 may also include various other types of equipment andsystems to provide a number of different functions. For example, asshown the payload 300 includes one or more communication systems 308,which may transmit signals via RF and/or optical links as discussedabove. The communication system(s) 308 include communication componentssuch as one or more transmitters and receivers (or transceivers), one ormore antennae, and a baseband processing subsystem. (not shown).

The payload 300 is illustrated as also including a power supply 310 tosupply power to the various components of balloon. The power supply 310could include one or more rechargeable batteries or other energy storagesystems like capacitors or regenerative fuel cells. In addition, theballoon 300 may include a power generation system 312 in addition to oras part of the power supply. The power generation system 312 may includesolar panels, stored energy (hot air), relative wind power generation,or differential atmospheric charging (not shown), or any combinationthereof, and could be used to generate power that charges and/or isdistributed by the power supply 310.

The payload 300 may additionally include a positioning system 314. Thepositioning system 314 could include, for example, a global positioningsystem (GPS), an inertial navigation system (INS), and/or astar-tracking system. The positioning system 314 may additionally oralternatively include various motion sensors (e.g., accelerometers,magnetometers, gyroscopes, and/or compasses). The positioning system 314may additionally or alternatively include one or more video and/or stillcameras, and/or various sensors for capturing environmental data. Someor all of the components and systems within payload 300 may beimplemented in a radiosonde or other probe, which may be operable tomeasure, e.g., pressure, altitude, geographical position (latitude andlongitude), temperature, relative humidity, and/or wind speed and/orwind direction, among other information. Wind sensors may includedifferent types of components like pitot tubes, hot wire or ultrasonicanemometers or similar, windmill or other aerodynamic pressure sensors,laser/lidar, or other methods of measuring relative velocities ordistant winds.

Payload 300 may include a navigation system 316 separate from, orpartially or fully incorporated into control system 302. The navigationsystem 316 may implement station-keeping functions to maintain positionwithin and/or move to a position in accordance with a desired topologyor other service requirement. In particular, the navigation system 316may use wind data (e.g., from onboard and/or remote sensors) todetermine altitudinal and/or lateral positional adjustments that resultin the wind carrying the balloon in a desired direction and/or to adesired location. Lateral positional adjustments may also be handleddirectly by a lateral propulsion system that is separate from thepayload. Alternatively, the altitudinal and/or lateral adjustments maybe computed by a central control location and transmitted by a groundbased, air based, or satellite based system and communicated to the HAP.In other embodiments, specific HAPs may be configured to computealtitudinal and/or lateral adjustments for other HAPs and transmit theadjustment commands to those other HAPs.

In order to affect lateral position and/or velocity changes, theplatform includes a lateral propulsion system. FIG. 4 illustrates oneexample configuration 400 of a balloon-type HAP with propeller-basedlateral propulsion, which may represent any of the balloons or otherlighter-than-air craft of FIG. 1. As shown, the example 400 includes anenvelope 402, a payload 404 and a down connect member 406 configured tocouple the envelope 402 and the payload 404 together. Cables or otherwiring between the payload 404 and the envelope 402 may be run within oralong the down connect member 406. One or more solar panel assemblies408 may be coupled to the payload 404 or another part of the balloonplatform. The payload 404 and the solar panel assemblies 408 may beconfigured to rotate about the down connect member 406 (e.g., up to 360°rotation or more), for instance to align the solar panel assemblies 408with the sun to maximize power generation. Example 400 also illustratesa lateral propulsion system 410. While this example of the lateralpropulsion system 410 is one possibility, the location could also before and/or aft of the payload section 404, or fore and/or aft of theenvelope section 402, or any other location that provides the desiredthrust vector. Details of the lateral propulsion system 410 arediscussed below.

Example Configurations

FIG. 5A illustrates an example 500 of the lateral propulsion system 410of FIG. 4. Example 500 includes a propeller assembly 502 affixed to acontrol assembly 504, as shown in FIG. 5B. The control assembly 504 isconfigured to manage operation of the propeller assembly 502, includingsetting its pointing direction, speed of rotation and determining whento turn on the propellers and for how long. As shown in FIG. 5A, thethree propeller blades of the propeller assembly 502 may be arrangedgenerally parallel to the down connect member 406. While three propellerblades are shown, two or more propeller blades may be employed. Thepropellers are able to rotate in either a clockwise or counterclockwisedirection as shown by arrow 506. The control assembly 504 is able torotate the propeller assembly about a longitudinal axis of the downconnect member 406 (e.g., up to or more than 360° rotation) as shown byarrow 508, changing the pointing direction of the propeller assembly 502in order to change the HAP's heading.

While this configuration or other similar configurations gives thelateral propulsion system 410 two degrees of operational freedom,additional degrees of freedom are possible with other pointingmechanisms or air-ducting mechanisms. This flexible thrusting approachmay be used to help counteract continually changing wind effects.Rotation of the control assembly 504 and propeller assembly 502 aboutthe down connect member 406 is desirably independent of rotation of thesolar panel assemblies (and/or payload) about the down connect member406.

FIGS. 6A-C provides enlarged views 600, 620 and 640, respectively, ofthe control assembly 504. The control assembly may include anelectronics module 602 for controlling operation of the assembly, acoupling section 604 that may be part of or otherwise connected to thedown connect member, and a propeller motor assembly 606. As shown, anouter cover or shroud 608 may encompass a cable management structure610, which is part of or secured to the coupling section 606. Power anddata cables can be run through the cable management structure 610, forexample connecting the electronics module 602 and other components ofthe lateral propulsion system to a power supply and/or control system ofthe payload. The cable management structure 610 is configured for powerand/or data cables to be placed in a helical arrangement, with theability to flex over a wide rotation range of the control assembly andpropeller assembly, e.g., up to 360°-400° or more, while providingdata/power to the lateral control system.

The payload or the lateral propulsion system (or both) may have on-boardsensors (e.g., differential GPS or DGPS) to provide accurate attitudeand/or position and velocity measurements, enabling highly accuratepointing of the propeller in an absolute direction as well as relativeto the payload direction. These sensors are also able to providemeasurement of the balloon platform's lateral speed. The propeller motorassembly 606 is configured to rotate the propeller in a clockwise orcounterclockwise direction, with or without additional gearing. Thepropeller motor assembly 606 may be brushless, which can generate moretorque than a brush-type motor. By way of example, the brushless motormay be a 1000W motor, which is capable of rotating the propeller at upto 2500 rpm or more. The motor may employ a cooling system, for instanceusing cooling fins or air ducts (not shown) to remove excess heat fromthe motor or electronics. The system may only need to drive thepropeller to achieve a lateral speed relative to the ground of between1-15 m/s in order to significantly increase the ability of the balloonto stay on or return to station. The speed may be dependent on thelocation of interest, wind currents at a particular location oraltitude, season/time of year, time of day, and/or other factors.

As shown in FIG. 6B, there may be a pointing axis motor assembly 622 inaddition to propeller motor assembly 606. The pointing motor assembly606 is configured to cause the control assembly and propeller assemblyto rotate about the down connect member. This may be done by actuating aworm gear mechanism 624. For instance, the pointing motor assembly 606may include a stepper or brushless DC motor that drives the worm gearmechanism 624, which enables the assembly to rotate about the downconnect member by up to 360°-400° or more. Rotation and pointing of thepropeller unit could be accomplished with many different configurationsof motors and gears or other mechanisms. Also shown in this figure is abraking mechanism 626, which can be used to stop rotation of thepropeller.

As shown in FIG. 6C, the electronics module 602 may include a controlsubsystem 642 and a power subsystem 644. A position sensor 646 may bepart of the position motor assembly 606, to determine a relativealignment of the propeller assembly relative to the down connect member.Adjacent to the propeller motor assembly 606 is the braking mechanism626, which may include a brake unit 648, a brake sensor 650, a holdingmagnet 652 and an actuator such as solenoid 654. These and otherfeatures of the brake mechanism 626 are discussed further below.

A block diagram of an exemplary electronics module 700 is illustrated inFIG. 7. The electronics module may be part of or separate from thenavigation system 316 or the control system 302 of the payload. Asshown, a CPU, controller or other types of processor(s) 702, as well asmemory 704, may be employed within the electronics module 700 to manageaspects of the lateral propulsion system. The power subsystem mayinclude a power usage controller 706 for managing various powersubsystems of the electronics module, including for altitude controlsystem (ACS) power 708 (e.g. to control buoyancy of the envelope), buspower 710, communication power 712 and lateral propulsion power 714. Thepower usage controller 706 may be separate from or part of theprocessor(s) 702.

The control subsystem may include a navigation controller 716 that isconfigured to employ data obtained from onboard navigation sensors 718,including an inertial measurement unit (IMU) and/or differential GPS,received data (e.g., weather information), and/or other sensors such ascomponent health and performance sensors 720 (e.g., a force torquesensor) to manage operation of the balloon's systems. The navigationcontroller 716 may be separate from or part of the processor(s) 702. Thenavigation controller works with system software, ground controllercommands, and health & safety objectives of the system (e.g., batterypower, temperature management, electrical activity, etc.) and helpsdecide courses of action. The decisions based on the sensors andsoftware may be to save power, improve system safety (e.g., increaseheater power to avoid systems from getting too cold during stratosphericoperation) or divert power to altitude controls or divert power tolateral propulsion systems.

When decisions are made to activate or make adjustments to the lateralpropulsion system, the navigation controller then leverages sensors forposition, wind direction, altitude and power availability to properlypoint the propeller and to provide a specific thrust condition for aspecific duration or until a specific condition is reached (e.g., aspecific lateral velocity or position is reached, while monitoring andreporting overall system health, temperature, vibration, and otherperformance parameters). In this way, the navigation controller cancontinually optimize the use of the lateral propulsion systems forperformance, safety and system health. Upon termination of a flight orindication of an emergent condition, the navigation controller canengage the safety systems (for example the propeller braking mechanism)to prepare the system to descend, land, and be recovered safely.

Lateral propulsion controller 722 is configured to selectively controlthe propeller's pointing direction, manage speed of rotation, powerlevels, and determine when to turn on the propeller or off, and for howlong. The lateral propulsion controller 722 thus oversees thrusterpointing direction 724, thruster power level 726 and thruster on-time728 modules. It can also manage a brake control module 730 to engageand/or disengage the braking mechanism. The lateral propulsioncontroller 722 and/or the brake control module may each be separate fromor part of the processor(s) 702. Processor software or received humancontroller decisions may set priorities on what power is available forlateral propulsion functions (e.g., using lateral propulsion power 714).The navigation controller then decides how much of that power to applyto the lateral propulsion motors and when (e.g., using thruster powerlevel 726). In this way, power optimizations occur at the overall systemlevel as well as at the lateral propulsion subsystem level. Thisoptimization may occur in a datacenter on the ground or locally onboardthe balloon platform.

The lateral propulsion controller 722 is able to control the drive motorof the propeller motor assembly so that the propeller assembly mayoperate in different modes. Two example operational modes are: powercontrol or rotational velocity control. The electronics module may storedata for both modes and the processor(s) of the control assembly maymanage operation of the drive motor in accordance with such data. Forinstance, the processor(s) may use the stored data to calculate orcontrol the amount of power or the rotational propeller velocity neededto achieve a given lateral speed. The electronics module may store datafor the operational modes and the processor(s) of the control assemblymay manage operation of the drive motor in accordance with such data.For instance, the processor(s) may use the stored data to calculate theamount of current needed to achieve a given lateral speed. Theprocessor(s) may also correlate the amount of torque required to yield aparticular speed in view of the altitude of the balloon platform.

The processor(s) may control the drive motor continuously for a certainperiod of time, or may cycle the drive motor on and off for selectedperiods of time, e.g., using pulse width modulation (PWM). This latterapproach may be done for thermal regulation of the drive motor. Forinstance, the propeller may be actuated for anywhere from 1 second to 5minutes (or more), and then turned off to allow for motor cooling. Thismay be dependent on the thermal mass available to dissipate heat fromthe motor.

As noted above, the lateral propulsion controller 722 regulates thethruster pointing direction 724, such as by causing the pointing motorassembly to drive the worm gear mechanism in a first direction to rotateclockwise about the down connect longitudinal axis or in a seconddirection to rotate counterclockwise about the longitudinal axis.

FIG. 8 illustrates a view 800 of an exemplary functional implementationof the lateral propulsion controller. In this example, external inputs802, such as control commands and/or balloon telemetry information(e.g., pressure rate, battery charge, etc.) are received and provided toone or more processors of the electronics module. For instance, a firstprocessor 804 may control operation of the pointing axis motor assemblyand a second processor 806 may control operation of the propeller motorassembly, which may include controlling operating of the brakingmechanism via the brake control module.

By way of example, a pointing control module may receive a pointing axisindex, which can indicate the pointing position of the propellerrelative to the down connect member, how many degrees of rotation thepropeller has moved relative to a default position, etc. In thisexample, such information is used by a stepper motor control module tocontrol operation of the pointing axis motor assembly, for instance byrotating in a clockwise (or counterclockwise) direction once a thresholdrotation amount has been exceeded (e.g., 320°) or a maximum rotationamount has been reached (e.g., 360° or 400°).

FIG. 9 illustrates a braking example 900 using the brake control module.As shown, hub 902 of the propeller assembly is coupled to a down connectmember 904 of the HAP via a transverse section 906, which may includethe propeller motor assembly. The brake mechanism 908 includes a brakepad 910. As indicated by arrow 912, the brake pad 910 can be driventowards the hub 902, using friction braking to cause the propellerassembly to stop rotating. The braking mechanism can be disengaged, andthe braking process repeated as needed during the operational lifetimeof the HAP. Further details of the brake mechanism and brake operationare discussed below.

Brake Mechanism

FIGS. 10A-C illustrate one example 1000 of a brake mechanism that may beused to rapidly bring the propeller assembly to a stop. The propellerassembly and propeller motor assembly (as seen in FIGS. 6A-C) areomitted for clarity. As shown in FIG. 10A, brake pad 1002 is receivedwithin a housing 1004. The brake pad 1002 is configured to linearlyextend from the housing 1004 (see arrow 912 of FIG. 9) in order tocontact the propeller hub to stop the rotation of the propellerassembly. The housing 1004 is coupled to support member 1006. Alsocoupled to the support member are a brake sensor 1008, a holding magnet1010 and an actuator such as solenoid 1012. An arm member 1014operatively couples the holding magnet 1010 and solenoid 1012 to thebrake pad 1002. Armature plate 1013 is shown as a round disc sandwichedbetween the holding magnet 1010 and the arm member 1014.

As seen in FIG. 10B, the housing 1004 includes a housing bracket 1016having an arcuate side, which is configured to mount adjacent to thecylindrical housing of the propeller motor assembly. And as seen in theview of FIG. 10C, a top (cover) section 1018 of the housing 1004 isaffixed to the housing bracket 1016 using a pair of bolts 1020. One ormore washers 1022 may be used as spacers between the bolt head and thetop section of the housing. By way of example, the washers 1022 may beBelleville washers, which are conical springs that are extremely stiffcompared to traditional coil springs. The washers' purpose is twofold.The first is to make the brake force ramp up instead of occurringsuddenly. The second is that the washers will increase the gap betweenthe housing 1004 and the housing bracket 1016 until the gap is too largeand force will stop increasing. A set of Belleville washers 1022 may bestacked all in the same direction or with adjacent washers in opposingdirections. Returning to FIG. 10A, a pair of pins 1024 (e.g., clevispins) are shown affixed to the housing 1004. The clevis pin will reachthe radius on 1002 and force will drop off. This is essentially a forcelimiter that prevents torque from going over a set value.

FIGS. 11A-B illustrate a configuration 1100 of the brake mechanism ofFIG. 10, but with the top of the housing 1004 removed. As shown in theperspective view of FIG. 11A and the top view of FIG. 11B, the armmember 1014 is coupled to the brake pad 1002 via a cable 1102. A spring1104 is disposed along the cable 1102 between the brake pad 1002 and thearm member 1014. The spring 1104 provides a small amount of force normalto the propeller hub. When the holding magnet is released, the springensures the brake pad makes contact with the hub. Once the pad 1002contacts the hub with a relatively small amount of normal force (fromthe spring), the friction between the pad and the hub will cause the padto move in the direction of the hub rotation and then bind against oneof the pins 1024, which are rolling member.

Thus, as shown in the cutaway view 1130 of FIG. 11C, the pad will eithermove in a first lateral direction as indicated by dashed arrow 1132 aalong a first one of the pins 1024 a, or move in a second lateraldirection as indicated by solid arrow 1132 b along a second one of thepins 1024 b. The partial see-through view of FIG. 11D illustrates asituation 1140 when the pad has moved to the second pin 1024 b. FIG. 11Eis a head-on view showing the pad moved to the second pin 1024 b. Thelateral displacement of the pad may be, e.g., on the order of 0.25 to1.5 inches, or more or less depending on the size of the arrangement.Once the angled surface of the pad has bound (wedged) between therotating hub and one of the pins, the normal braking force applied tothe hub is proportional to the wedge angle of the brake pad, frictionbetween the brake pad and the hub, and the kinetic energy of thespinning propeller assembly. The pins 1024 help re-align the pad to acentered position once the brake is disengaged.

FIG. 11F illustrates a view 1150 in which the pad has been laterallydisplaced for engagement with the hub (not shown). Here, the Bellevillewashers 1022 are in a compressed state, and there are gaps 1152 and 1154between certain components (for 1154, this is a gap between the armatureplate 1013 and the holding magnet 1010). In contrast, as seen in view1160 of FIG. 11G, when the pad has been disengaged, the Bellevillewashers 1022 are in an uncompressed state. In this case, there is no gapbetween the components as indicated by arrows 1162 and 1164.

FIG. 12 illustrates a view 1200 showing the brake pad 1002, cable 1102and spring 1104 in isolation. The surface area and shape of the brakepad contact surface may be chosen based on the configuration of thepropeller hub, the amount of braking force desired, and other factors.The braking surface of the brake pad can be smooth, rough or grooved.For instance, a rough surface may have pits and raised portions. Thedifferent surface types may not have a noticeable impact on the brakingprocess, but may affect galling and friction between the brake pad andthe propeller hub. In certain implementations it may be beneficial tominimize or avoid galling.

FIG. 13 illustrates an example 1300 of the arm member 1014. As shown,the arm member in this example includes a first extension 1302 thatcouples to the holding magnet and a second extension 1304 that has afirst receptacle 1306 a for a magnet used as part of the brake sensorand a second receptacle 1306 b. Between the two extensions is a centralregion 1308, which couples to the solenoid. Extending from the centralregion 1308 is an arm 1310, which includes a receptacle 1312 thatreceives the cable 1102. And as shown in view 1400 of FIG. 14, magnet1402 is affixed to receptacle 1306 a. A sensor 1404, such as a HallEffect sensor, is slightly spaced apart from the magnet 1402 to detect astrength of the magnetic field, which is used to detect a relativedisplacement of the arm member 1404. A pin, bar or other shaft-typeelement 1406 is partly disposed in the receptacle 1306 b. The arm memberslides on the shaft element 1406, which is used to keep the arm assemblyaligned with the solenoid, holding magnet and Hall Effect sensor. Thecable 1102 that connects the arm assembly to the brake pad isintentionally flexible, thereby allowing the brake pad to move laterallyin the direction of hub rotation to bind against the pin as discussedabove. The lateral movement of the brake pad places a small lateral loadon the arm, which the shaft element 1406 resists. Maintaining axialalignment of the solenoid plunger and coil winding may be important sothat the solenoid plunger does not bind on the solenoid housing when thesolenoid is activated to disengage the brake

Brake Operation

During normal operation of the propeller, the brake pad is disengagedfrom the propeller hub so that the propeller may provide lateralpropulsion to the HAP. Upon determination of descent of the HAP,uncontrolled propeller rotation, power loss or situations when thepropeller is not being used, the system causes the brake pad to engagewith the propeller hub. In this configuration, power is applied to thesystem when the brake pad is disengaged. In the event of power loss, thebrake system is automatically engaged

According to one aspect of the technology, the magnet 1402 and HallEffect sensor 1404 are used to detect whether the brake pad is engagedwith the propeller hub. The actuation distance between the magnet andsensor is small, and an analog feedback signal from the sensor providesa controller (e.g., processor 806 of FIG. 8) with a distance delta thatis used to determine whether or not the brake is engaged. In someexamples, the distance may be, e.g., on the order of 1-2 mm, less than10 mm, etc. For instance, in one scenario the distance between thesurface of the brake pad and the hub is between 0.75-1.5 mm in thedisengaged state and 0 mm for the engaged state. Other techniques couldalternatively be used to detect whether the brake pad is engaged withthe hub, such as evaluating a motor status signal.

In an alternative configuration, an electromagnet may be employed todetect brake engagement. Here, the magnetic field of an energizedelectromagnet would change depending on if a ferrous material is incontact with it or not. In this case, the Hall Effect sensor would beplaced where the highest delta in the magnetic field occurs, and thecontroller could detect whether or not the electromagnet is engaged withthe armature plate (1013 in FIG. 10a ) holding the brake pad back.

As noted above, according to one scenario the brake is always engagedwhen the propeller system is idle. Here, brake would only be disengagedwhen the propeller is running. Thus, applications of the braking systemalways assume the propeller is currently spinning. The brake mechanismmay be actuated to stop the propeller assembly in different ways. In oneexample, actuation logic of the controller responds to changes inpressure events, tilt, and overspeed of the motor. For instance, thebraking logic may map certain conditions to severity of the brakingresponse. Some examples include (i) high tilt angle of the balloonenvelope (e.g., in excess of 10-30 degrees of tilt, or more or less),(ii) high change in ambient pressure (e.g., in excess of a 10-25% changein pressure), (iii) moderate fan (propeller) overspeed (e.g., on theorder of 5-20% overspeed or no more than 30% overspeed), and (iv) highfan (propeller) overspeed (e.g., on the order of 25-50% overspeed, atleast 20% overspeed, etc.). For a high tilt condition, the propellermotor may be stopped and the brake is applied when the propeller hasstopped spinning. For a high change in ambient pressure, the brake maybe applied immediately. For moderate fan overspeed, the motor may bestopped and the brake applied when the propeller has stopped spinning.And for high fan overspeed, the brake may be applied immediately.

FIG. 15A illustrates a state diagram 1500 showing different states ofthe braking process, for instance as performed by brake control module730. Engaged state 1502 is when the brake is engaged with the propellerhub. The system may continuously or systematically monitor forunexpected disengagements in the unexpected disengage state 1504.Release state 1506 includes the process for releasing the brake, whichcan include energizing both the electromagnet and the solenoid. Judgingstate 1508 may include the system determining whether the brake hassufficiently pulled away from the hub, for instance so that the lateralpropulsion system may be activated. Disengaged state 1510 is when thebrake is fully disengaged from the hub so that the propeller can freelyrotate for lateral propulsion. And the system may continuously orsystematically monitor for unexpected engagements in the unexpectedengage state 1512.

In another approach, information from the Hall Effect sensor may be usedas follows. The Hall Effect sensor outputs of the motor increase infrequency as the motor speed increases. These sensor output can be feedinto a counter that is reset periodically by an external real time clockevent. When the counter exceeds a predefined maximum count rating beforethe periodic periodically reset, the brake power would be disengaged.Alternatively, the energy from each rising edge of the sensor outputswould be AC coupled and accumulated. Here, whenever the accumulationtrips beyond a setpoint in a comparator, the comparator would toggle anddisengage the brake power. In one scenario, power is disconnected andthe brake is applied any time the propeller is not running, which savespower.

Prior to engagement, an actuation technique may be used to ensure thebrake is disengaged. The first stage energizes the solenoid (1012 inFIG. 10A) to pull back the braking surface of the brake pad 1002 awayfrom the propeller hub. The second stage de-energizes the solenoid 1012and energizes the electromagnet 1010, which holds the braking surfacefrom engaging. This provides a high steady state force to power ratio.When power to the magnet is turned off, the magnetic field collapses andthe spring 1104 engages the brake pad 1002, causing the brake pad tocontact the propeller hub.

FIG. 15B illustrates an example 1550 of a brake disengagement actuationprocess that may be performed by brake control module 730, for instancebased upon the lateral propulsion controller 722 planning to activatethe propeller assembly. Block 1552 starts from the brake engaged statein which the brake pad physically arresting the propeller hub (theEngaged state). At block 1554, the electromagnet is first energized toallow time for the magnetic field to build up (part of the Releasestate). Then at block 1556, the solenoid is energized to pull back thebrake pad from the propeller hub (part of the Release state). In oneexample, the solenoid may be energized with a substantially higher thanrated power to save weight. The tradeoff to this approach is that thesolenoid can only be actuated for a short duration before it must bepowered off to cool down. If this time limit is hit before the brakefeedback reports the brake pad is pulled back, the state would go toJUDGING, where solenoid is powered de-energized and a short time isallowed for the brake feedback to report it is pulled back. If itsucceeded, the state would go to DISENGAGED, while if it failed, itwould go back to ENGAGED and reports an error.

Next, at block 1558 the system waits for feedback to show the brake ispulled back (part of Release and Judging states). Once feedback (e.g.,from the Hall Effect sensor) shows the brake has been pulled back, thesolenoid is de-energized at block 1560 (the Disengaged state). Then thebrake is disengaged at block 1562, and the electromagnet is de-energizedat block 1564. The process then is able to repeat as shown by dottedarrow from block 1564 to block 1552. There may also be a fallbackoperating mode for situations where the brake feedback path has failed.In this case, the solenoid is always actuated for the maximum duration,and every pull-back attempt is assumed to be successful.

Additionally, the firmware monitors for the brake engaging anddisengaging without being commanded. For instance, block 1566 indicatesthat if there is an unintended or unexpected disengagement, an errorsignal may be raised. Similarly, block 1568 indicates that if there isan unintended or unexpected engagement, an error signal may also beraised. With regard to energizing the solenoid, as shown at block 1570the system may determine that the energy limit is reached in block 1570.In response, the solenoid and electromagnet may be de-energized at block1572. If there is a failure to release, then at block 1574 an errorsignal can be raised. These operations may cover failure modes such asexternal debris pulling the brake open, or holding electromagnet, aswell as other situations.

This braking approach may be repeated as needed during flight. Forinstance, it may be employed multiple times to prevent free spinning ofthe propeller blades, and then one time during final descent of the HAPback to earth. As mentioned above, without such a braking approach itmay take tens of seconds, minutes or longer for the propeller blades tostop rotating. Furthermore, the propeller would almost certainly speedup from dynamic pressure on descent. The approaches presented herein maycause the blades to stop in a few seconds or less. For instance, FIG. 16presents a plot 1600 of velocity (rpm) 1602 and torque 1604 versus time.As shown, once the brake pad engages the propeller hub, the measuredtorque increases rapidly as the velocity decreases rapidly. In thisplot, just prior to 2500 ms, while the velocity continues to decreaselinearly the torque begins to drop. And around the 3000 ms mark, whenthe velocity is very low (about 250 rpm versus more than 1250 rpm at2000 ms) the torque quickly decreases from about 37 NM to almost 0 Nm bythe 3700 ms mark. In this example, once the torque begins to increase,the velocity drops to 0 rpm within approximately 1500 ms. The reasonthat the torque experiences a knuckle from the rapid increase to a slowdecrease to a rapid decrease is due to a beneficial property of thebrake pad. When the rpms of the propeller are low, kinetic coefficientof friction dominates where most of the braking torque is generatedfrom. When the rpms of the propeller are high, galling between the brakepad surface and propeller hub dominate where most of the braking torqueis generated. This is beneficial because the kinetic coefficient offriction is mostly constant across increasing rpms; however, gallingincreases with increasing rpms. This leads to increasing brake torquefor increasing rpms. Put simply, the higher the rpms and/or the higherthe rotation force, the higher the braking force. However, above acertain rpms level, such as around 2000 or so, increasing rpms does notincrease braking torque.

In one scenario, the solenoid uses a control loop to pull back the brakewith constant power. This provides a pulling force independent ofbattery voltage. It can also provide consistent heat generation, whichis worst at ambient temperature, allowing for easier qualification. Thisis because the solenoid winding resistance is highest at warmertemperatures. In addition, this type of control loop is able to providea higher pull force for the same power consumed at flight temperatures,providing additional margin. This is because the solenoid winding islowest at colder temperatures, which improves efficiency.

FIG. 17 illustrates an example method 1700 for controlling lateralpropulsion in a HAP-type device that is configured to operate in thestratosphere. At block 1702, a brake control module causes anelectromagnet of a brake assembly of the high altitude platform toenergize. At block 1704, the brake control module causes solenoid of thebrake assembly of the high altitude platform to energize. At block 1706,the brake control module judges, based on the energized electromagnetand the energized solenoid, that a brake pad of the brake assembly hasdisengaged from contact with a hub of a propeller assembly. And at block1708, in response to the judging that the brake pad of the brakeassembly has disengaged, the system causes the propeller assembly tospin at a selected rate in a selected direction to cause the highaltitude platform to move in a specified direction.

And FIG. 18 illustrates another example method 1800 for controllingpropeller operation in a high altitude platform configured to operate inthe stratosphere. At block 1802, the method includes determining, by abrake control module, that a brake pad of a brake assembly is disengagedfrom contact with a hub of a propeller assembly of the high altitudeplatform. The propeller assembly is configured to spin at a selectedrate in a selected direction when the brake pad is disengaged in orderto cause the high altitude platform to move in a specified direction. Atblock 1804, the brake control module determines whether a selectedcondition has occurred with respect to the high altitude platform. Andat block 1806, in response to determining that the selected conditionhas occurred, the brake control module causes the brake pad to engagewith the hub of the propeller assembly to cause the propeller assemblyto cease rotation.

The foregoing examples are not mutually exclusive and may be implementedin various combinations to achieve unique advantages. As these and othervariations and combinations of the features discussed above can beutilized without departing from the subject matter defined by theclaims, the foregoing description of the embodiments should be taken byway of illustration rather than by way of limitation of the subjectmatter defined by the claims. In addition, the provision of the examplesdescribed herein, as well as clauses phrased as “such as,” “including”and the like, should not be interpreted as limiting the subject matterof the claims to the specific examples; rather, the examples areintended to illustrate only one of many possible embodiments. Further,the same reference numbers in different drawings can identify the sameor similar elements.

1. A brake mechanism configured to stop rotation of a propellerassembly, the brake mechanism comprising: a brake pad configured toengage with a hub portion of the propeller assembly to stop the rotationof the propeller assembly; a housing receiving at least part of thebrake pad therein during disengagement from the propeller assembly; abrake sensor configured to detect whether the brake pad is engaged withor disengaged from the hub portion of the propeller assembly; and anactuator assembly including a magnet and an actuator, the actuatorassembly configured to maintain the brake pad in a disengaged stateduring a first operational mode and to cause the brake pad to engage thehub portion of the propeller assembly during a second operational mode.2. The brake mechanism of claim 1, wherein the actuator assembly furtherincludes a spring coupling the brake pad to the actuator, the springconfigured to provide a spring force to cause the brake pad to engagethe hub portion of the propeller assembly during the second operationalmode.
 3. The brake mechanism of claim 2, wherein the actuator assemblyfurther includes an arm member coupled between the actuator and thespring.
 4. The brake mechanism of claim 3, further comprising a cableaffixed to the brake pad and the arm member, wherein the spring isdisposed around at least part of the cable.
 5. The brake mechanism ofclaim 3, wherein the arm member includes a first extension affixed tothe magnet, a second extension opposite the first extension, and acentral region disposed between the first extension and the secondextension, the central region affixed to the actuator.
 6. The brakemechanism of claim 5, further comprising: a second magnet affixed to thesecond extension; and a Hall Effect sensor in operative communicationwith the second magnet affixed to the second extension, the Hall Effectsensor configured to detect the strength of a magnetic field associatedwith the second magnet to detect a relative displacement of the brakepad.
 7. The brake mechanism of claim 1, wherein the actuator is asolenoid.
 8. The brake mechanism of claim 1, wherein the housingincludes a housing bracket and a cover section affixed to the housingbracket, the housing bracket and the cover section defining a receptacleto at least partly receive the brake pad.
 9. The brake mechanism ofclaim 8, wherein the housing bracket includes an arcuate side configuredto abut a motor assembly that drives the propeller assembly.
 10. Apropulsion system for use with a high altitude platform configured tooperate in the stratosphere, the propulsion system comprising: apropeller assembly including a plurality of propeller blades and centralpropeller hub affixed to the plurality of propeller blades; a motorassembly operatively coupled to the central propeller hub of thepropeller assembly, the motor assembly being configured to actuate thepropeller assembly to drive the high altitude platform in a lateraldirection in the stratosphere; and a brake mechanism configured to stoprotation of the propeller assembly, the brake mechanism comprising: abrake pad configured to engage with the propeller hub of the propellerassembly to stop the rotation of the propeller assembly; a housingreceiving at least part of the brake pad therein during disengagementfrom the propeller assembly; a brake sensor configured to detect whetherthe brake pad is engaged with or disengaged from the propeller hub ofthe propeller assembly; and an actuator assembly including a magnet andan actuator, the actuator assembly configured to maintain the brake padin a disengaged state during a first operational mode and to cause thebrake pad to engage the propeller hub of the propeller assembly during asecond operational mode.
 11. The propulsion system of claim 10, whereinthe actuator assembly further includes a spring coupling the brake padto the actuator, the spring configured to provide a spring force tocause the brake pad to engage the propeller hub of the propellerassembly during the second operational mode.
 12. The propulsion systemof claim 11, wherein the actuator assembly further includes an armmember coupled between the actuator and the spring.
 13. The propulsionsystem of claim 12, the brake mechanism further comprising a cableaffixed to the brake pad and the arm member, wherein the spring isdisposed around at least part of the cable.
 14. The propulsion system ofclaim 12, wherein the arm member includes a first extension affixed tothe magnet, a second extension opposite the first extension, and acentral region disposed between the first extension and the secondextension, the central region affixed to the actuator.
 15. Thepropulsion system of claim 14, further comprising: a second magnetaffixed to the second extension; and a Hall Effect sensor in operativecommunication with the second magnet affixed to the second extension,the Hall Effect sensor configured to detect a strength of a magneticfield associated with the second magnet to detect a displacement of thebrake pad relative to the propeller hub.
 16. The propulsion system ofclaim 10, wherein the actuator is a solenoid.
 17. The propulsion systemof claim 10, wherein the housing includes a housing bracket and a coversection affixed to the housing bracket, the housing bracket and thecover section defining a receptacle to at least partly receive the brakepad.
 18. The propulsion system of claim 17, wherein the housing bracketincludes an arcuate side configured to abut a housing of the motorassembly.
 19. A high altitude platform configured to operate in thestratosphere, the high altitude platform comprising: a balloon envelope,a payload; a connecting member coupling the payload to the balloonenvelope; and the propulsion system of claim 10, wherein the propulsionsystem is operatively engaged with the connecting member.
 20. The highaltitude platform of claim 19, further comprising a control systemconfigured to actuate the brake mechanism for engagement with anddisengagement from the propeller hub in response to a predeterminedcondition.